Abstract: Designing an efficient and reliable rainwater harvesting and reuse system requires comprehensive consideration of hydrology, structure, process engineering, and intelligent control. Based on hands-on experience from hundreds of projects, this article answers the seven most frequently asked questions in design, aiming to provide clear decision-making references for engineers and project owners.
1. How to scientifically determine the volume of a rainwater storage tank?
This is the starting point of design. Too small a volume, and rainwater cannot be effectively collected; too large, and investment increases. The core principle is “sizing storage for usage, while also considering retention.”
- Reuse volume: Prioritize user demand. According to standards (e.g., China GB50400), the effective volume of the storage tank should be approximately the average 3-day reclaimed water demand. For example, if a project requires 30 tons per day for landscape irrigation, the recommended reuse storage volume is no less than 90 cubic meters.
- Retention volume: In Sponge City projects, the tank must also contribute to total runoff control. Calculate the required total retention volume based on the local annual runoff control rate target, then coordinate the allocation between the reuse tank and other facilities like rain gardens and bio-retention areas.
2. How to correctly select PP modular tanks and design for load‑bearing?
Module selection directly affects underground structural safety and cannot be based on price alone.
- Selection criteria: Choose based on cover depth, surface loading (green space, parking stall, or fire lane), and local geological conditions. For example, for small parking areas with cover depth less than 2 meters, high‑strength modules (e.g., LDX‑PP‑7) can be used. For deeper cover or locations under roadways, side‑load‑reinforced modules (e.g., LDX‑PP‑9) are required, possibly combined with civil measures like concrete side walls.
- Safety reminder: Always perform uplift calculation. When cover depth is shallow (e.g., <1.5m), the tank may float; design must specify anti‑flotation measures.
3. How to accurately determine sizing parameters for lift pumps and supply pumps?
Pumps are the “heart” of the system. Wrong selections lead to poor operation or high energy consumption.
- Flow calculation:
- Dewatering (drain) pump flow rate ≈ tank volume / 12 hours.
- Supply pump flow rate = daily water demand / (4–8 hours). For small to medium projects, consider multiples of 5 m³/h.
- Treatment lift pump flow rate ≥ supply pump flow rate.
- Head calculation: Total head = net lift height + pipe friction & minor losses + equipment resistance + free outlet head. As a quick estimate: pipe losses ≈ (net head + friction loss) × 1.2~1.3. For example, for a large campus project (>10,000 m²), a supply pump total head of 40–45 meters is typical.
4. Why are smart level sensors more reliable than traditional float switches?
Traditional mechanical floats are prone to sticking and failure, causing control malfunction. Smart level sensors use piezoresistive or ultrasonic principles and output a continuous 4‑20 mA analog signal to the PLC.
- Advantages: They allow precise setting of any start/stop level, enabling complex control logic (e.g., pump alternation, sleep/wake). The signal is stable over distances of 100–200 meters and includes self‑diagnostics, making them standard in modern smart water systems.
5. How to configure a filter backwash system for both efficiency and energy savings?
Backwash is critical to maintain filtration performance; improper configuration leads to incomplete cleaning or water waste.
- Dedicated backwash pump: Configure an independent pump for backwash. The backwash line only needs one electric/solenoid valve – simple control.
- Using the supply pump for backwash: When the supply pump is also used for backwash, a pressure‑reducing valve must be installed in series on the backwash line to prevent excessive pressure from damaging the filter media. This saves one pump but involves slightly more complex logic.
- Energy‑saving highlight: Using air‑water combined backwash technology significantly improves cleaning effectiveness and reduces water consumption (can be less than 1% of treated water volume).
6. UV disinfection vs. chlorination – how to choose?
Each has its appropriate application, depending on water quality requirements, operational conditions, and safety considerations.
- UV disinfection: Physical, instant inactivation, no chemical residue, safe and environmentally friendly. Suitable for rainwater and greywater reuse where residual chlorine is not required and water clarity is good. It is the mainstream choice in integrated smart units.
- Chlorination: Chemical, provides a residual disinfectant effect to maintain chlorine levels at the pipe network endpoints. Suitable for systems requiring continuous disinfection or dealing with complex water quality, but involves management of hazardous chemicals (sodium hypochlorite).
7. How to achieve true “intelligence” and low maintenance in the system?
Intelligence is not merely remote control – it is the system’s ability to be self‑aware and self‑governing.
- Core: A PLC‑based controller integrated with online monitoring of smart level, pressure, and water quality parameters (e.g., SS, residual chlorine).
- Advanced functions:
- Variable‑frequency constant‑pressure water supply: Automatically adjusts pump speed based on demand, saving up to 30% energy and protecting equipment.
- Predictive maintenance: Analyzes pump running current, vibration, or filter pressure differential trends to issue alerts before failure occurs.
- IoT platform: Data uploaded to the cloud, enabling remote monitoring via mobile/PC, report analysis, and smart scheduling anywhere in the world – achieving “unattended operation with supervised management.”
Conclusion
An excellent rainwater system is the result of precise calculation, quality component selection, and intelligent control. Based on a decade of deep experience, Ledo Industrial has distilled the above knowledge into standardized design tools and a full range of high‑performance products, committed to making every rainwater reuse project a model of reliability, efficiency, and intelligence.